Folded filament beam generator



March 2, 1965 c, w. HANKS ETAL 3,172,007

EQLDED F ILAMENT BEAM GENERATOR Filed Jan. 15, 1962 5 K M #mm Miv Z Wm (M QHHUH a \HV ITT'GP/Vi j United States Patent 3,172,007 FOLDED FILAMENT BEAM GENERATOR Charles W. Hanks, Orinda, and Maurice E. Tyler, El

Cerrito, Calif., assignors, by mesne assignments, to

Stauffer Chemical Company, New York, N.Y., a corporation of Delaware Filed Jan. 15, 1962, Ser. No. 166,255 7 Claims. (Cl. 315-14) The present invention relates to an improved electron beam generator for high energy application, wherein very high current electron beams are required such as, for example, in electron beam furnaces.

Extensive investigation and development in the field of electron sources have produced major improvements therein, and it is possible with conventional electron source configurations to produce almost any desired beam configuration or control. While the present invention generally pertains to this same field of endeavor, it differs therefrom in that the invention hereof is directed to the production of extremely high energy beams that may be employed, for example, in electron beam furnaces or the like in order to supply suflicient energy therein for the bombardment, melting, and casting of materials. The very high electron emission required for this type of application causes difficulty in beam formation as very large magnetic fields are produced by the heating currents and these fields deflect emitted electrons. A serious disadvantage of incomplete control over high current beam lies in the undesirable bombardment of surrounding equipment with any uncontrolled portion of the emission. With very high current electron emission, the random propagation of even a small percentage thereof is quite hazardous, inasmuch as sumcient electron energy would then be expended in random bombardment to materially affect and possibly destroy surrounding equipment.

Although it is quite conventional to employ accelerating means in order to collimate electron emission into an electron beam and thus to direct such a beam to a desired focus, it has been found that this alone does not suffice to prevent the propagation of copious quantities of electrons in relatively random directions when very large emission currents are involved. The present invention provides for initially directing emitted electrons in a desired direction away from the emission area so that the emitter itself co-operates with accelerating means in the establishment of a desired beam geometry, and furthermore, provides for materially simplifying the control over very high current beams. The foregoing is accomplished herein through the utilization of a double or folded filament configuration, wherein a pair of electron emissive elements are disposed in close parallelism with electrical currents flowing in opposite directions in these two elements. Thermionic emission thus produced from the elements is caused to proceed in the particular direction therefrom in part by the magnetic fields established by these two portions of the folded filament hereof. The folded filament beam source of the present invention has proven to be highly advantageous in electron beam furnaces employed in the melting of refractory metals and is, of course, applicable to other high energy beam applications.

The present invention is illustrated as to a particular preferred embodiment thereof in the accompanying drawing wherein:

FIGURE 1 is a perspective view of an electron beam generator in accordance with this invention;

FIGURE 2 is a representation of magnetic field lines about the emitters of the source of the present invention;

FIGURE 3 is a schematic illustration of an electron beam furnace including the electron beam generator of FIGURE 1;

3,172,007 Patented Mar. 2, 1965 FIGURE 4 is a transverse sectional view through a modified version of the electron beam generator of this invention; and

FIGURE 5 is a schematic illustration of alternative electrical connections to separate electron emitters in accordance with this invention.

The present invention, in brief, comprises a backing electrode defining an elongated emission chamber having an opening to the exterior of the electrode. Within this chamber there are disposed a pair of spaced parallel electron emitters which are joined together at one end thereof to form a U-shaped filament. An electric current is passed serially through these emitters of the filament to raise same to a sufficient temperature for electron emis sion and this current produces magnetic fields surrounding the emitters. Inasmuch as the current passes in oppo-,

site directions through the two emitters, there is thus produced a magnetic field configuration, wherein the magnetic fields combine in additive relation between the emitters. This magnetic field configuration establishes a virtual source of electrons along the line half way between and parallel to the two emitters, with electrons traveling from such line in the direction of the lines of force of the combined magnetic fields. The above-described filament is disposed within the emission chamber of the backing electrode with the direction of electron emission pointed toward an opening in such chamber. Exteriorly of the chamber there is provided electronic accelerating means establishing an electric field attracting electrons emitted from the filament. It is found that with this configuration substantially all electrons leaving the filament area travel in the same direction, as constrained by the magnetic fields about the filament emitters, and consequently, the formation of the emission into an electron beam is greatly facilitated.

Considering now the present invention in somewhat greater detail and referring to the drawing, there will be seen to be illustrated in FIGURE 1 a broken perspective view of a preferred embodiment of the present invention and including a backing electrode 11 defining a generally cylindrical emission chamber 12 therein. This chamber communicates with the exterior of the backing member at a relatively large opening longitudinally of the member and chamber. Immediately above a lower lip 13 of the backing member 11 and within the chamber 12, there is disposed a filament 14 comprising first and second parallel emitters 16 and 17. The emitters 16 and 17 of the fila-' ment may, for example, be formed of 70 mil tungsten Wire and are joined together at one end thereof to thereby form a U-shaped configuration, as illustrated. The two emitters of the filament are arranged in closely spaced parallelism with the separation between emitter wires being of the order of the diameter of an individual wire. Suitable insulating supporting means are provided for mounting the filament in the position shown and described.

Externally of the backing electrode 11, there is disposed an electron accelerating electrode 19 which extends longitudinally of the element with a leading edge of such electrode adjacent and below the lip 13 upon the backing element. Electron emission from the filament 14 is accomplished by energization of this filament, and such is illustrated schematically by the power supply 21 connected across the free ends of the two emitters 16 and 17. An electronic accelerating field is established by suitable energization of the accelerating electrode 19, and this is schematically illustrated by the battery 22, shown in FIG- j URE l as having the positive side thereof electrically connected to the electrode and the negative side thereof returned to the backing electrode and filament as through ground connections.

The passage of a current through the emitters of the filament 14 produces a magnetic field encircling each of these emitters. The present invention is directed to the production of very high energy electron beams and, consequently, there is employed a very high heating current which is passed through the emitters in order to achieve the desired quantity of electron emission. Consequently, the strength of the magnetic fields encircling the emitters is actually many orders of magnitude greater than is normally encountered in electron sources. The magnetic field configuration about the emitters of the filament is schematically illustrated in FIGURE 2, wherein the separate emitter wires 15 and 17 are shown as being encircled by magnetic field lines, indicated by the dashed lines thereabout. Inasmuch as current flows in opposite directions through the two emitters, it will be seen that the magnetic field lines extend, for example, in a clockwise direction about the lower emitter 1'7 and in a counter clockwise direction about the upper emitter 16. This produces an addition of magnetic fields between the emitters, and as illustrated, the magnetic field lines are directed from left to right between the emitters.

Production of the desired magnetic field configuration, noted above, requires passage of electrical current in opposite directions through the two emitters. Thus, in the instance wherein the emitters are separately energized, the power supply is connected oppositely to each emitter. This connection is shown in FIGURE 5 wherein separate emitters 16 and 17 are shown to be connected in opposite sense across a power supply 21 so that currents flow through the emitters to establish the field configuration of FIGURE 2.

Electrons thermionically emitted from the emitters 16 and 17 are acted upon by an accelerating electric field established between the accelerating electrode 19 and the filament and backing electrode so as to be attracted away from the filament. As these emitted electrons travel away from the emitters they cross lines of magnetic forces shown in FIGURE 2, and consequently, are acted upon by forces at right angles to the direction of travel and to the direction of the magnetic field lines. Although the overall magnetic envelope about the filament is seen to resemble the magnetic field configuration about an individual wire, the actual fields existing within this envelope differ quite materially from that about a single wire. In the instance wherein emission is produced from a single wire, it has been found in instances of very high filament currents that the electrons are unduly affected to deflect the trajectory of same, and consequently, to produce a majority of electron emission at an angle toward one end of the emission wire. In the present invention this undesirable defiection is prevented inasmuch as electrons having a component of velocity longitudinally of the emitters pass through magnetic fields of Opposite senses, and consequently, receive a substantially net zero defiection in this direction. Furthermore, with regard to the direction of electrons from the filament hereof, it is noted that the intensity profile of the magnetic field in a plane through both of the emitters of FIGURE 2 exhibits a double hump with a valley existing half way between the emitters. Emitted electrons are urged toward the area of lower field strength and trapped. As a consequence of this field configuration, electrons are constrained to travel in a plane which is perpendicular to the plane of the two emitters and equal distance between these emitters. This is indicated by the large black arrow 31 in FIGURE 2, wherein it is assumed that an electron accelcrating field from acceleration means urges electrons to the right in this figure. This magnetic collimation of emitted electrons is highly desirable and advantageous inasmuch as it materially simplifies the direction of electrons into a beam. Generation of an actual beam of electrons is highly desirable and important in many applications and particularly with regard to the present invention, wherein it is desired that the electron energy shall be 4 most fully utilized as for the heating and melting of materials such as refractory metals, for example.

Although the problems of electron beam generation have been widely investigated, it is particularly pointed out in connection with the present invention that there is produced hereby an electron beam of extremely high energy, wherein a very dense beam is generated. Very high filament currents are employed for this generationand the present invention utilizes the magnetic fields estab-' lished about the emitters of the filament to define the direction of electron emission while at the same time preventing the strong magnetic fields produced from undesirably deflecting the emitted electrons. With very high beam energies it becomes particularly important to limit, if not preclude, even small percentages of random emission. The escape of some small percentage of an electron beam having a current of the order of amperes produces considerable damage when such escaped portion of the beam bombards elements in the vicinity of the beam generator, for example. Relatively rapid destruction of gen erator structure or other apparatus in the path of high current random emission most certainly occurs. The present invention prevents this occurrence by the limitation of emission direction, and experimentation has shown that substantially no electrons are emitted at large angles to the plane extending through the emitters of the filament.

The very high energy and high density electron beam generated by the present invention is particularly useful in electron beam furnaces such as illustrated in FIGURE 3. As shown therein, an enclosure 41 is evacuated to a very low pressure, as indicated by the block arrows 42, and a melt stock 43 is arranged to be lowered in the enclosure above a mold 44. The electron beam generator of the present invention, as indicated at 46, is disposed to direct a very high energy electron beam 47 into the open top of the mold 44 and the melt stock 43 is fed into this beam so that the beam bombards a portion of the melt stock, and consequently, melts it so that it drips downwardly into the mold. Within the mold this molten material is further bombarded and heated by the electron beam focused to generally bombard all of the open top' of the mold. This mold 44 may be formed, for example, of copper with cooling tubes therein for the passage of water in order to prevent damage to the mold itself and also in order to remove heat from the lower portion of the material dripping into the mold so that a solidified ingot 48 is formed beneath the molten pool of material at the top of the mold. This ingot 48 may be continuously removed or pulled downwardly from the bottom of the mold.

Electron beam furnaces of the type generally described above are known in the art, and it is quite important that electron beams employed therein shall have a very high energy in order to melt the melt stock and to further heat the molten pool of material within the mold. It is also quite necessary for the electron beam to be rather sharply defined in order that it shall only bombard the melt stock as it is moved into the beam, rather than providing a variable or a random bombardment of it. Likewise it is necessary for the beam definition to be adequate to prevent undue bombardment of the top of the mold, for quite clearly this would then destroy the mold itself. The electron beam generator of the present invention does provide for the generation of an electron beam having the above-noted requirements. Extremely dense electron beams are produced in accordance with the present invention, and the electron beam appears to be generated at a point or line between the parallel emitters of the filament. A divergence of the beam during its traverse from the generator to the area of bombardment will be seen to be illustrated in FIGURE 3, and this is a natural consequence of the repulsion existing between the electrons of the beam. This beam divergence is not only acceptable herein but is also highly desirable, for an expanded beam focus at the top of the mold then serves to properly heat all of the molten pool of material therein.

Various modifications of the present invention are, of course, possible and there is illustrated in FIGURE 4 a transverse sectional view of an electron beam generator in accordance with the present invention and including an additional element in the form of a movable front plate 51. This plate is physically and electrically connected to the backing electrode 11 in extension in part across the opening of the emission chamber 12 therein. The front plate 51 may be mounted as by means of bolts 52 upon the front of the backing member, and slots 53 provided in this front plate allow for vertical control over the position of this front plate. Consequently, it will be seen that the electric field configuration at the entrance to the emission chamber 12 is controllable by varying the position of the front plate 51. There is established a curved electric field between the backing element and accelerating electrode, and the electrons emitted from the filament 14 are acted upon by this curved electric field to thereby follow a curved trajectory in traveling outwardly of the emission chamber. This is illustrated by the dashed lines in FIGURE 4. The curvature of the electric field in this vicinity is adequate to curve the electrons emanating from the emission chamber, but the electric field strength is not sufficient in this area to direct ions back into the emission chamber. This is particularly important in applications of the electron beam generator hereof, wherein substantial quantities of ions may be present in the vacuum chamber within which the generator is disposed. Thus, for example, in the casting and purification of metals in an electron beam furnace, there may be produced copious quantities of ions through bombardment of the metal itself and also through ionization of various vapors and gases evolved during the course of processing. Inasmuch as the accelerating electrode 19 is maintained at a relatively positive potential with respect to the backing electrode, it will be seen that ions reaching the vicinity of the opening of the emission chamber will then be acted upon by an electric field accelerating the ions into the emission chamber. Inasmuch as the filament is disposed out of line with a direct path into the emission chamber, this filament is then protected from ion bombardment. Although ions may be slightly curved in passage through this accelerating field, they will not be acted upon sufliciently to curve them into impingement with the emitters of the filament. Instead ions will at most enter the emission chamber and bombard the rear portion thereof. For this reason the backing electrode 11 may be formed of a relatively massive structure to readily accommodate any such ion bombardment.

The folded filament structure of the present invention has been described above with respect to particular preferred embodiments thereof and in connection with a preferred application thereof, wherein sufiicient electron beam energy is produced to rapidly melt even refractory metals in large quantities. Attainment of electron emission of this order may be accomplished by the passage of a current of the order of 115 to 125 amperes through a 70 mil tungsten filament including two parallel emitters as described above. Application of a kilovolt accelerating voltage, for example, produces an extremely energetic electron beam highly suitable for utilization in electron beam furnaces.

A very close spacing of the filament emitters is desirable in order to achieve maximum direction of the emitted electrons, and as above noted, a separation of the filaments by a distance of the order of the filament diameter has proven advantageous. With the magnitude of heating current employed there are produced magnetic fields of the order of hundreds of gauss immediately adjacent the emitters, and consequently, there results a strong repulsive force between the separate parallel emitters of the filament. Care must be taken to avoid bowing of the filament wires, for any distortion of the wires produces a distortion of the magnetic fields thereabout, and consequently, an unpredictable variation in the direction of electron trajectory.

Extreme bowing of the wires may actually cause them to touch and thus possibly to even short out a part of the emissive portion of the filament. In placement of the filament within the backing electrode or element it is important in order to attain all of the advantages of the present invention to insure a line of sight between at least one of the emitters and the leading edge of the accelerating electrode. Although it is possible to attract electrons from the filament with a different arrangement than that stated above, it has been found through experimentation that maximum directive effects are achieved when the lip 13 of the backing electrode does not completely mask the accelerating electrode from the filament.

Various modifications and variations in the abovedescribed embodiments of the present invention are possible. Thus, for example, the electron beam generator hereof may be elongated in the form of an annulus, preferably with an opening in the backing electrode directed outwardly of same, and this configuration is quite advantageous in certain electron beam furnace applications wherein it is desired to direct electrons into the open top of a casting mold from a plurality of directions. It is also possible to employ externally generated magnetic fields for additional direction of the electron beam generated hereby. With such additional beam control it is possible to turn the beam through almost any angle desired and thus in connection with electron beam furnaces, it is possible to dispose the electron beam generator hereof beneath the top of the mold in the furnace and to direct the electron beam generally upwardly into a magnetic field hav ing lines of force normal thereto for constraining the beam to traverse a curved trajectory above the mold and downwardly into the open top thereof. In this instance the beam may even be initially directed from the generator radially outward and somewhat upwardly of the mold, so as to provide even greater isolation of the generator itself from ion bombardment.

Although the present invention has been described in connection with particular preferred embodiments thereof, it is not intended to limit the invention by the terms of this description nor by the showing in the drawings. Instead reference is made to the appended claims for a precise delineation of the present invention.

What is claimed is:

1. An improved electron beam source comprising a backing element defining a chamber therein with a longitudinal opening to the exterior of said backing element, a pair of electron emitters disposed in spaced parallel relation within said chamber along said opening and in a plane across said opening, means connecting said emitters together at one end of each, means passing a current through said emitters in such a direction as to establish thereby a magnetic field having field lines extending toward said opening between said emitters, and electron accelerating means disposed exteriorly of said backing element adjacent the longitudinal opening therein, whereby eleetrons emitted from said emitters are directed through said opening.

2. An improved electron beam generator comprising a pair of electron emitters disposed in closely spaced parallel relationship and electrically joined together at one end of each to form a U-shaped electron source, means passing current serially through said emitters to establish electron emission therefrom, said current further establishing magnetic fields having field lines extending in opposite directions about said emitters to thereby add together between the emitters, a backing member defining an emission chamber opening to the exterior thereof and disposed about said emitters in position to direct said adding magnetic field lines outwardly of the member through the chamber opening, and electron acceleration means adjacent said member outside said chamber for establishing an electron accelerating field whereby electrons emitted from said emitters substantially all travel along said adding magnetic field lines for establishing an electron beam.

3. An improved electron source for emitting very large quantities of electrons comprising first and second elongated electron emitters disposed in closely spaced parallel relationship, means passing a very large current through one of said emitters and a like very large current in an opposite direction through the other emitter for establishing thermionic emission therefrom, said current establishing magnetic fields encircling said emitters with lines of force of the fields adding between the emitters, and elongated electron accelerating means disposed longitudinally of said emitters in spaced parallel relation thereto adjacent the space therebetween, whereby the establishment of an electron accelerating field withdraws emitted electrons along a plane intermediate the emitters and perpendicular to a common plane of the emitters.

4. An electron source as set forth in claim 3 further defined by said emitters having the configuration of straight wires joined together at one of the adjacent ends of each, and said means passing electric current serially through said emitters to thereby establish said magnetic fields.

5. An improved electron beam generator adapted for disposition in an evacuated chamber and comprising a filament having two parallel emitting wires in closely spaced and insulated relationship, connections for the passage of a heating current through the filament to raise at least the emitter wires to a temperature for thermionic emission and to establish a strong magnetic field about said wires which adds together between same, and means establishing an electron accelerating field adjacent said filament for attracting electrons therefrom in a plane between the wires and parallel thereto.

6. An improved electron beam generator comprising a folded filament having two elongated electron emissive portions disposed in closely spaced parallel relation, connections to said filament for passing a large heating current through said filament along one filament portion in a direction opposite to heating current flow in the other portion, an accelerating electrode adapted to establish an electron accelerating field extending to said filament for attracting emitted electrons therefrom, and means at least in part extending between said filament and accelerating electrode longitudinally thereof,- said means being adapted to be maintainedat substantially the potential of said filament for bowing said accelerating field about said means whereby emitted electrons are withdrawn in quantity from said filament only along lines of force of said accelerating field.

7. An improved electron source for emitting very large quantities of electrons comprising an electron emitter including a single U-shaped emitter wire having a pair of elongated parallel legs disposed in closely spaced parallel relationship and separated by a distance substantially equal to the diameter of the wire, and means passing a very large current through the wire of said electron emitter to establish electron emission therefrom, said current establishing magnetic fields encircling the wire of said emitter with the lines of force of the fields adding between the legs of the emitter, whereby the establishment of an electron accelerating field withdraws emitted electrons along a plane intermediate the emitters and perpendicular to a common plane of the emitters.

References Cited in the file of this patent UNITED STATES PATENTS 2,090,722 Bouwers Aug. 24, 1937 2,471,298 Arlee May 24, 1949 2,880,483 Hanks et al Apr. 7, 1959 2,994,801 Hanks Aug. 1, 1961 FOREIGN PATENTS 1,100,200 Germany Feb. 23, 1961 OTHER REFERENCES Sears and Zemansky, College Physics, 3rd edition, 1960, Chapter 33, Section 4, page 645. 

6. AN IMPROVED ELECTRON BEAM GENERATOR COMPRISING A FOLDED FILAMENT HAVING TWO ELONGATED ELECTRON EMISSIVE PORTIONS DISPOSED IN CLOSELY SPACED PARALLEL RELATION, CONNECTIONS TO SAID FILAMENT FOR PASSING A LARGE HEATING CURRENT THROUGH SAID FILAMENT ALONG ONE FILAMENT PORTION IN A DIRECTION OPPOSITE TO HEATING CURRENT FLOW IN THE OTHER PORTION, AN ACCELERATING ELECTRODE ADAPTED TO ESTABLISH AN ELECTRON ACCELERATING FIELD EXTENDING TO SAID FILAMENT FOR ATTRACTING EMITTED ELECTRONS THEREFROM, AND MEANS AT LEAST IN PART EXTENDING BETWEEN SAID FILAMENT AND ACCELERATING ELECTRODE LONGITUDINALLY THEREOF, SAID MEANS BEING ADAPTED TO BE MAINTAINED AT SUBSTANTIALLY THE POTENTIAL OF SAID FILAMENT FOR BOWING SAID ACCELERATING FIELD ABOUT SAID MEANS WHEREBY EMITTED ELECTRONS ARE WITHDRAWN IN QUANTITY FROM SAID FILAMENT ONLY ALONG LINES OF FORCE OF SAID ACCELERATING FIELD. 